In order to provide aircraft operators with sufficient high lift performance in critical stages of a flight, aircraft wings have been designed to have deployable and retractable slats along the leading edge. This has historically been achieved by mounting the slats on pairs of curved slat tracks, which are driven from their respective stowed and extended positions by a rack and pinion mechanism. Aircraft wings are used to store the fuel required for the flight and as such maximising the volume available for the fuel is important. For this reason the front spar is generally positioned as far forward as possible, leaving minimal space within the wing leading edge to house the various systems and devices which must reside there.
These are two conflicting requirements: a large leading-edge area to house moveable devices and a large fuel volume. The resulting solution has been to allow the slat tracks to penetrate the front spar (fuel boundary) by adding cylindrical track cans which are attached to the rear of the front spar for the tracks to extend into when in the stowed (retracted) position. It is therefore critical to note that for this reason the track cans are also part of the fuel boundary wing box structure.
A typical aircraft wing assembly incorporating such a track can is shown in FIG. 1. The wing has a relatively long slat track 8 carrying a slat 9 which is driven by a pinion gear along a curved path defined by a set of rollers between a retracted low-lift cruise position, an intermediate take-off and climb position, and a fully extended high-lift landing position. FIG. 1 shows the slat in both extreme ends of its movement. As the slat track 8 moves, it slides in and out of a slat track can 16 which is attached to the rear face of the web of the front spar 17.
Track cans are traditionally fabricated as a welded aluminium assembly, which is a notoriously expensive manual process requiring skilled personnel a long time to fabricate. This combination invariably results in a high level of scrap and hence greater expense.
Track cans also present significant problems from a mounting point of view. Integral to the welded construction is a flange at the base of the track can with captive nuts used for attachment to the spar. Each track can (of which there are often more than twelve on each wing) must be fitted and sealed to preserve fuel integrity. This is time consuming and subject to leakage, resulting in greater expense.
Further to these problems are the issues which arise when track cans are used within composite wing box construction. The inherent differences between the mechanical properties of composites when compared to metals results in a reduction in ‘flat’ spar web area, for a given outer wing profile, to which a track can is able to be mounted. To compound this problem further is the shift towards shallower wing boxes i.e. shallower spar webs. Combining these two problems results in conflicts between track cans and other wing box components.
Similar problems exist in relation to other system components which are normally housed in the leading or trailing edge of an aircraft wing: for instance transformers for an electrical de-icing system, or actuator units for driving a slat deployment mechanism. As with a slat track it would also be desirable to house such components on the “fuel” side of the spar web in order to free up space within the leading or trailing edge.
Although the above discussion has focused on the aircraft wing, similar problems also exist in relation to other aerodynamic parts of the aircraft such as the vertical tail plane and horizontal tail plane.
A so-called “Sine-Wave Spar” is described in “Composite Airframe Structures—Practical Design Information and Data”, Page 234, Michael C. Y. Niu, ISBN 962-7128-06-6, Published 1992. The spar web is formed with sinusoidal corrugations which extend from the spar web at a relatively shallow angle.